1. Field of the Invention
The present invention relates to a pencil lead composition and also to an advantageous method for manufacturing such lead.
2. Description of the Prior Art
Pencil lead, for example, black lead, has heretofore been manufactured by blending a coloring material such as natural graphite or artificial graphite, with a clay binder, grinding the blend with water and kneading it, extruding the wet material so obtained into the appropriate form for a lead pencil, and after drying, baking it at elevated temperatures of about 1,000.degree. C and further oil-immersing it. Such a method is complicated for a manufacturing process not only in that it involves a large number of steps but also because close control of the water content at the time of extrusion is required. Moreover, it is necessary to bake the flexible extrudate while maintaining it in a straight line. However, it is almost impossible to bake an endless length of lead continuously. Thus, there is an inherent limitation to the number and types of variations for such a manufacturing process.
In order to eliminate or improve upon these defects in the conventional pencil lead making process, various methods of obtaining lead without need for baking have been proposed. These methods use various kinds of thermoplastic or thermosetting synthetic resins as a binder and involve blending and kneading them with a colored material and extruding the blend through a screw type extruder. (For example, see Japanese Patent Publication Gazette Nos. 3363/1952, 2309/1967, 13368/1973, 18900/1973, 12300/1974 and 12302/1974, and Patent Public Disclosure Gazette Nos. 16214/1972 and 55017/1974. )
However, the leads obtained by these methods are significantly inferior in performance to conventional lead obtained by using clay as a binder and baking at elevated temperatures. Particularly, the H, F, HB and B classes of lead, which are in great demand, suffer from the defect that a finely shaved point is easily broken because these leads have insufficient strength.
Generally, the writing depth (or density) of lead and its strength are inversely related to one another; that is, the deeper the depth, the lower the strength. In lead blended with a resin as a binder, the strength increases directly with the amount of resin component but the depth decreases therewith. Although, in lead below 2H in depth, e.g., 3H, 4H, 5H, etc., there is no problem in practical applications with use of various synthetic resins, as described in the above-mentioned publications, the demand for pencil lead is mostly concentrated in the vicinity of the HB level. Therefore, if a lead of depth as high as HB cannot be made of high strength in a given process, the method suffers enormously from an industrial viewpoint. Consequently, it would be most desirable to have an approximately HB pencil lead of high strength which is fabricated using a synthetic resin binder.